Complex Catchment Processes that Control Stream Nitrogen and Organic Matter Concentrations in a Northeastern USA Upland Catchment
There is a need to understand the coupled biogeochemical and hydrological processes that control stream hydrochemistry in upland forested catchments. At watershed 9 (W-9) of the Sleepers River Research Watershed in the northeastern USA, we use high-frequency sampling, environmental tracers, end-member mixing analysis, and stream reach mass balances to understand dynamic factors affect forms and concentrations of nitrogen and organic matter in streamflow. We found that rates of stream nitrate processing changed during autumn baseflow and that up to 70% of nitrate inputs to a stream reach were retained. At the same time, the stream reach was a net source of the dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) fractions of dissolved organic matter (DOM). The in-stream nitrate loss and DOM gains are examples of hot moments of biogeochemical transformations during autumn when deciduous litter fall increases DOM availability. As hydrological flowpaths changed during rainfall events, the sources and transformations of nitrate and DOM differed from baseflow. For example, during storm flow we measured direct inputs of unprocessed atmospheric nitrate to streams that were as large as 30% of the stream nitrate loading. At the same time, stream DOM composition shifted to reflect inputs of reactive organic matter from surficial upland soils. The transport of atmospheric nitrate and reactive DOM to streams underscores the importance of quantifying source variation during short-duration stormflow events. Building upon these findings we present a conceptual model of interacting ecosystem processes that control the flow of water and nutrients to streams in a temperate upland catchment.
Source-to-Stream Connectivity Assessment Through End-member Mixing Analysis
Streamflow sources across various hydrologic conditions were examined in a 5.1 ha temperate humid forested catchment (Laurentians, Canada). In that system, high discharges can be associated with both low and high antecedent conditions, while high antecedent conditions do not always result in high discharges. Examining the temporal persistence of streamflow sources is therefore crucial for internal catchment connectivity, given the assumption that their geochemical signature is time-invariant and that only their presence and mixing in streamflow vary with time. Multiyear daily stream chemistry data were broken down into several hydrologic scenarios reflecting different conditions with respect to stream discharge and antecedent catchment wetness. For each hydrologic scenario, eigenvector and residual analysis of stream chemistry were performed to meet the objectives of: (1) estimating the dimensionality of the mixing space; (2) evaluating the fit of independently sampled end-members (throughfall, organic and mineral soil water in riparian, organic, and upslope locations + catchment-wide baseflow) in the mixing space to assess internal catchment connectivity from a spatial standpoint; and (3) carrying hydrograph separation to examine internal catchment connectivity from a volumetric standpoint. Mixing space dimensionality did not vary significantly among the tested hydrologic scenarios, as three end-members were generally required to account for most of the variance in stream geochemistry. Differences were however significant in the ability of the tested end-members to fit in mixing spaces. All independently sampled throughfall and organic soil water end-members better fitted in mixing spaces associated with high discharges, in comparison to mixing spaces associated with low discharges. The relative contributions of end-members to streamflow were highly variable across times. On the one hand, scenarios involving low discharges and dry antecedent conditions were mostly associated with baseflow, while scenarios involving low discharges and wet antecedent conditions were associated with contributions from both baseflow and organic soil water originating from midslope locations. On the other hand, scenarios involving high discharges and wet antecedent conditions were associated with increased proportions of throughfall and organic soil water from riparian and upslope areas. These results suggest a cautious evaluation of the predictive power of one single mixing space with regards to the nature of streamflow sources across hydrologic conditions.
Effects of Urbanization on Stream Water Quality in the City of Atlanta, Georgia, USA
A long-term stream water-quality monitoring network was established in the City of Atlanta (COA) during 2003 to assess baseline water-quality conditions and the effects of urbanization on stream water quality. Routine hydrologically-based manual stream sampling, including several concurrent manual point and equal width increment sampling, was conducted approximately 12 times per year at 21 stations, with drainage areas ranging from 3.7 to 232 km2. Eleven of the stations are real-time (RT) water-quality stations having continuous measures of stream stage/discharge, pH, dissolved oxygen, specific conductance, water temperature, and turbidity, and automatic samplers for stormwater collection. Samples were analyzed for field parameters, and a broad suite of water-quality and sediment-related constituents. This paper summarizes an evaluation of field parameters and concentrations of major ions, minor and trace metals, nutrient species (nitrogen and phosphorus), and coliform bacteria among stations and with respect to watershed characteristics and plausible sources from 2003 through September 2007. The concentrations of most constituents in the COA streams are statistically higher than those of two nearby reference streams. Concentrations are statistically different among stations for several constituents, despite high variability both within and among stations. The combination of routine manual sampling, automatic sampling during stormflows, and real-time water-quality monitoring provided sufficient information about the variability of urban stream water quality to develop hypotheses for causes of water-quality differences among COA streams. Fecal coliform bacteria concentrations of most individual samples at each station exceeded Georgia's water-quality standard for any water-usage class. High chloride concentrations occur at three stations and are hypothesized to be associated with discharges of chlorinated combined sewer overflows, drainage of swimming pool(s), and dissolution and transport during rainstorms of CaCl2, a deicing salt applied to roads during winter storms. Water quality of one stream was highly affected by the dissolution and transport of ammonium alum [NH4Al(SO4)2] from an alum manufacturing plant in the watershed; streamwater has low pH (<5), low alkalinity and high concentrations of minor and trace metals. Several trace metals (Cu, Pb and Zn) exceed acute and chronic water-quality standards and the high concentrations are attributed to washoff from impervious surfaces.
Nutrient Cycling in Streams: Hydrologic and Biogeochemical Controls on Intra-Annual Temporal Fluctuations
Rapid changes in land use (e.g., shifts from agriculture to urban; intensification of agriculture) and projected impacts of global climate change (e.g., shifts in precipitation patterns) are expected to be reflected in shifts in the hydrologic cycles and the biogeochemical cycles of nutrients and other contaminants, with an attendant spatial and temporal perturbations in aquatic ecosystem functions. Episodic nutrient loads to lotic systems occur via runoff events, with the advective transport along the river network moderated by biogeochemical transformations, thus mitigating the likely impacts on downstream ecosystems. Exploring two types of patterns are of interest when examining nutrient cycling rates in river networks: (1) spatial patterns in temporally integrated (e.g., annual average) nutrient losses along the river networks, and (2) temporal patterns (e.g., intra- or inter-annual) in spatially integrated trends in nutrient losses at a given location within the network. The first type has received considerable attention over the past two decades in a series of field studies and modeling studies to interpret these data. However, studies focusing on temporal variations have only recently begun to receive attention. In examining nutrient losses along the stream/river networks, it is important to acknowledge the importance of: (1) the spatial patterns emerging from the hydrologic controls on biogeochemical processes as the contributing drainage area increases; and (2) the temporal variations reflective of the short-term (intra- annual) and long-term (inter-annual) variations in climate-hillslope-vegetation interactions reflected in stream flow variations. We present here a conceptual framework of hierarchical levels of linked hydrologic and biogeochemical controls, and then present a stochastic analytical approach to explicitly link such interactions to predict the intra-annual temporal variations in nutrient cycling in streams. Available data for nitrate cycling in stream networks is examined as the first case study for this approach.
Dissolved Organic Matter and Emerging Contaminants in Urban Stream Ecosystems
We investigated the effects of urbanization on the sources, bioavailability and forms of natural and anthropogenic organic matter found in streams located in Maryland, U.S.A. We found that the abundance, biaoavailability, and enzymatic breakdown of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP) increased in streams with increasing watershed urbanization suggesting that organic nutrients may represent a growing form of nutrient loading to coastal waters associated with land use change. Organic carbon, nitrogen, and phosphorus in urban streams were elevated several-fold compared to forest and agricultural streams. Enzymatic activities of stream microbes in organic matter decomposition were also significantly altered across watershed land use. Chemical characterization suggested that organic matter in urban streams originated from a variety of sources including terrestrial, sewage, and in-stream transformation. In addition, a characterization of emerging organic contaminants (polyaromatic cyclic hydrocarbons, organochlorine pesticides, and polybrominated diphenyl ether flame retardents), showed that organic contaminants and dissolved organic matter increase with watershed urbanization and fluctuate substantially with changing climatic conditions. Elucidating the emerging influence of urbanization on sources, transport, and in-stream transformation of organic nutrients and contaminants will be critical in unraveling the changing role of organic matter in urban degraded and restored stream ecosystems.
Linking Near Real-Time Water Quality Measurements to Fecal Coliforms and Trace Organic Pollutants in Urban Streams
Anthropogenic pollutants, including pesticides, herbicides, pharmaceuticals, and estrogens are detected in urban water bodies. Effective examination of dilute organic and microbial pollutant loading rates within surface waters is currently prohibitively expensive and labor intensive. Effort is being placed on the development of improved monitoring methodologies to more accurately assess surface water quality and evaluate the effectiveness of water quality management practices. Throughout the summer and fall of 2008 a "real-time" wireless network equipped with high frequency fundamental water quality parameter sensors measured turbidity, conductivity, pH, depth, temperature, dissolved oxygen and nitrate above and below stormwater inputs at two urban stream locations. At each location one liter grab samples were concurrently collected by ISCO automatic samplers at two hour intervals for 24 hour durations during three dry periods and five rain events. Grab samples were analyzed for fecal coliforms, atrazine (agricultural herbicide), prometon (residential herbicide) and caffeine (wastewater indicator). Surrogate relationships between easy-to-measure water quality parameters and difficult-to-measure pollutants were developed, subsequently facilitating monitoring of these pollutants without the development of new, and likely costly, technologies. Additionally, comparisons were made between traditional grab sampling techniques and the "real-time" monitoring to assess the accuracy of Total Maximum Daily Load (TMDL) calculations.
Spatial Scaling Patterns of C, N and P Loads in Engineered Watersheds: Hydrologic vs. Biogeochemical Drivers
Understanding nutrient dynamics in diverse ecosystems is critical in evaluating ecological impacts (e.g., eutrophication; coastal hypoxia) from increased loads of nitrogen (N), phosphorus (P), and carbon (C). The linkage between the hydrologic and the biogeochemical cycles is crucial for predicting nutrient cycling in these ecosystems. Examining the impacts of large-scale human modifications of watersheds (e.g., land-use intensification for food production; hydrologic modification though extensive tile-drainage, etc.) on the hydrologic and biogeochemical responses, and ecological impacts at various scales has been the focus of large-scale monitoring and modeling studies over the past two decades. Non-linear interactions between the climate (rainfall, evapotranspiration) and landscape are modified by the fractal river network to generate emergent scaling patterns of runoff that has been studied in considerable detail. The role of biogeochemistry as an additional non-linear filter that modifies the runoff signature to generate emergent patterns of nutrient loads has received much less attention. While scaling behavior of streamflow has been observed to be a function of the time scales of rainfall and catchment response, scaling patterns of nutrient loads would also be dependent on the time scales of the contaminant input function, and reaction time scales within various components of the system (hillslope, riparian zones, stream network). We examined the hydrologic and water-quality monitoring data available for the Mississippi River Basin, and found consistent linear relationships between area-normalized annual discharge (Q; L3L-2T-1) and area- normalized annual nutrient loads (ML-2T-1) at all spatial scales, ranging from first-order watersheds (~101 to 102 km2) to the entire river basin (~3x106 km2). By comparing the load-discharge data for conservative constituents (e.g., bicarbonate) with that for more-reactive constituents (nitrate, phosphate, pesticides), we estimated the effective attenuation rate constants at each spatial scale. We derived explicit analytical expressions for reproducing the reported scale-dependence of the nutrient attenuation rate constants. Finally, we used a simple hillslope-network model to investigate the spatial scaling patterns of nutrient loads as a function of the transport and reaction time scales. Implications of these results to predicting water quality impacts of land-use and climate change are discussed.
Metal Cycling in Polymictic Suburban Retention Ponds
Stratified conditions in lakes have been demonstrated to enhance metal species mobilization as well as the potential for mercury methylation. However, few studies have been conducted in shallow engineered systems. Although each system is relatively small in area, the overall number of such engineered systems is large (and increasing) and warrants consideration within overall landscape nutrient cycling. Previous research has documented strong diel stratification cycles and the frequent development of anoxia within the bottom waters of such polymictic systems compared with larger, dimictic lakes. We examined the impact of polymixis and the shorter hydraulic residence time on the bioavailability and the downstream transport of Hg species and other trace metals. Filtered and unfiltered lake water samples were collected at 15 and 50 cm above the sediment as well as the surface of the 1-m deep Mirror Lake retention pond on the University of Connecticut Storrs campus. Additional samples were collected from the lake outlet under baseflow and elevated discharge conditions, including the capture of initial mobilization during precipitation events. Samples were analyzed for Hg speciation as well as dissolved organic carbon (DOC), total suspended solids, cations (including Cu, Zn and Pb) and anions. We measured stage height at the lake outlet to calculate flux. Lake total Hg (THg) concentrations were generally less than 4 ng/L with the majority in the particulate phase. Outlet THg increased to 32 ng/L and dissolved THg increased to 1.2 ng/L during high flow events likely due to enhanced mobilization of particulates from the sediment and runoff from impervious surfaces, respectively. In contrast, DOC concentrations decreased as runoff contributions increased and were not correlated with dissolved THg. In addition, THg concentrations increased following copper algaecide applications, possibly due to re- suspension in the water column of biotic material.